A class of brownmillerite-derived materials have been found to be mixed ionic and electronic conductors (MIEC). See: PCT/US96/14841, filed Sep. 13, 1996. Gas-impermeable membranes made from MIECs behave as short-circuited electrochemical cells, in which oxygen anions and electrons conduct in opposite directions through the membrane. The oxygen anion conductivity of these materials make them useful for the efficient generation of pure oxygen which can be employed in catalytic oxidation reactions. The electronic conductivity of these materials provides for spontaneous gas phase separation and any subsequent oxidation without the addition of external electronic circuitry. MIEC membranes have found application in catalytic membrane reactors (CMRs) for a variety of processes where oxygen reacts with inorganic or organic species, such as hydrocarbons, including but not limited to the partial oxidation of methane and other hydrocarbons to synthesis gas, the oxidative coupling of aliphatic and aromatic hydrocarbons, and the gasification of coal. CMR processes can also include decomposition of organic environmental pollutants such as PCBs. These membranes can simply be used for the separation of oxygen from oxygen-containing gas (e.g., air) and the production of pure oxygen.
In operation, the ceramic membranes of CMRs are exposed to net oxidizing and reducing atmospheres on opposite sides of the membrane. The stresses imposed on the ceramic membranes arise from chemical expansion as well as thermal expansion. The membranes may also be exposed to significant pressure differentials up to about 500 psi. While reactor design can be optimized to minimize these stresses, brittleness and cracking of membrane materials after short term operation is a significant problem. Thus, there is a need in the art for ceramic membrane materials, particularly for long-term reactor operation, that retain excellent ionic and electronic conductivities and which, in addition, exhibit improved mechanical strength under reactor operating conditions.
This invention relates to ceramic compositions for use in ceramic membranes which contain multiple crystalline phases wherein the predominant phase is a MIEC. The ceramic compositions contain one or more second crystalline phases distinct from the MIEC phase which do not significantly contribute to ionic or electronic conductivity, but which enhance the mechanical strength of the composition. Mechanical strength of membrane made of these mixed phase materials is enhanced, particularly under CMR operating conditions. The presence of one or more second phases in a ceramic material significantly enhances the mechanical strength of the composition for use as a membrane in CMR applications. MIEC materials in general can be strengthened by the addition of second phases to decrease their brittleness and facilitate their practical application in CMRs. The mechanical strength of MIEC membranes that contain mixed ionic and electronic conducting brownmillerite-derived ceramics are particularly benefitted by the introduction of second phases.
In preferred ceramic compositions, the MIEC phase of the ceramic materials of this invention represents about 80 wt % or more of the composition. In more preferred compositions, the MIEC phase represents about 90 wt % or more of the composition. Lower levels (up to about 5 wt %) of second phases can arise as impurities during the preparation of MIEC materials, because of impurities in starting material resulting in inaccuracies in measurement of metal stoichiometries prior to reaction. Second phases can be selectively added to MIEC up to about 20 wt % by mixing components in off-stoichiometric amounts, i.e., by adding additional amounts of one or more metal precursors. The amount of second phase(s) which can be present in the final ceramic products (membranes) and improve their mechanical properties can vary most generally between about 0.1 wt % and about 20 wt %.
More specifically, mixed phase ceramic materials combining an MIEC phase from about 80 wt % to about 99 wt %, and a second phase or phases from about 1 wt % to about 20 wt % are useful for producing improved membranes of this invention.
Ceramic membranes of this invention can have any shape, form or dimensions suitable for use in various CMRs. In particular, membranes can be tubes of various diameters and lengths and flat plates or disks of various diameters. Ceramic membranes can be substantially composed of mixed phase ceramic as a dense material with membrane thicknesses ranging from about 0.5 to about 2 mm. Alternatively, gas impermeable membranes can be composed of a porous substrate supporting a dense thin film of the mixed phase ceramic, typically having a film thickness of about 1 xcexcm to about 300 xcexcm, more preferably having a thickness of 10 xcexcm to about 100 xcexcm.
Preferred mixed phase materials of this invention contain a sufficient amount of second phase to provide enhanced mechanical properties to the material (compared to a corresponding single-phase MIEC material) without significant detrimental effect upon the ionic and electronic conductivity of the mixed phase material. More specifically, in preferred mixed phase materials, the amount of the second phase(s) present is optimized to reduce breakage from handling and cutting in the preparation of membranes and to reduce breakage and cracking of membranes or thin films associated with thermal and chemical shock to the membrane during CMR operation. Further, the second phase(s) preferably do not substantially react with reactive gases during CMR operation to cause decomposition and failure of the membranes.
Mixed phase materials of this invention include those that contain about 1 wt % or more of second phases, those that contain about 2-4 wt % or more of second phases and those that contain 5 wt % or more of second phases. Preferred second phases are pseudo-binary and pseudo-ternary phases formed from the same (or fewer) elements as the mixed conductors. The second phases need not be ionic or electronic conductors and they need not be in thermodynamic equilibrium with the MIEC materials.
MIEC brownmillerite-derived ceramics are of general composition:
AXAxe2x80x2Xxe2x80x2Axe2x80x32xe2x88x92(x+xxe2x80x2)BYBxe2x80x2yxe2x80x2Bxe2x80x32xe2x80x94(y+yxe2x80x2)O5+z, 
where A is an element from the f block lanthanide elements; Axe2x80x2 is an element selected from the Group 2 elements, Axe2x80x3 is an element from the f block lanthanide or Group 2 elements; B is an element selected from Al, Ga, In or mixtures thereof; Bxe2x80x2 and Bxe2x80x3 are different elements and are selected independently from the group of elements Mg, and the d-block transition elements, including Zn, Cd, or Hg; 0 less than x less than 2, 0 less than xxe2x80x2 less than 2, 0 less than y less than 2, 0 less than yxe2x80x2 less than 2 where x+xxe2x80x2xe2x89xa62 and y+yxe2x80x2xe2x89xa62, and z varies to maintain electroneutrality. Axe2x80x3 and Bxe2x80x3 may or may not be present.
The lanthanide metals include the f block lanthanide metals: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Yttrium has properties similar to the f block lanthanide metals and is also included herein in the definition of lanthanide metals. A is preferably La or Gd, with La more preferred. Group 2 metal elements of the Periodic Table (also designated Group 2a) are Be, Mg, Ca, Sr, Ba, and Ra. The preferred Group 2 elements for the Axe2x80x2 element of the materials of this invention are Ca, Sr and Ba and Sr is most preferred. The more preferred B elements are Ga and Al, with Ga more preferred. The d block transition elements include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn. Preferred Bxe2x80x2 and Bxe2x80x3 elements are Mg, Fe and Co, with Fe and Co being more preferred as Bxe2x80x2 and Bxe2x80x3, respectively.
The value of z in the above formula depends upon the values of x, xxe2x80x2, y and yxe2x80x2 and the oxidation states of the A, Axe2x80x2 Axe2x80x3, B, Bxe2x80x2 and Bxe2x80x3 elements. The value of z is such that the mixed metal oxide material is charge neutral. In preferred materials, 0 less than z less than 1.
In preferred MIEC materials of this formula 0.05 less than x less than 1, 1 less than xxe2x80x2 less than 1.95, 0.25  less than y less than 1.2, and 0.8  less than yxe2x80x2 less than 1.75. In more preferred MIEC materials of this formula 0.2 less than x less than 0.6, 1.4 less than xxe2x80x2 less than 1.8, 0.6 less than y less than 1.0, and 1.0 less than yxe2x80x2 less than 1.4.
Second phases employed in the ceramic materials of this invention improve the mechanical strength of the MEIC materials and are not significantly detrimental to their electrical properties. Second phases are structurally distinct from the MIEC phases. Second phases can include phases of the AB2O4 structure type, such as SrAl2O4, and the A2BO4 structure type, such as Sr2AIO4. More generally, second phases can include the quaternary and ternary phases (A, Axe2x80x2)2 (B, Bxe2x80x2)O4, Axe2x80x22 (B, Bxe2x80x2)O4, (A, Axe2x80x2)(B, Bxe2x80x2)2O4, Axe2x80x2(B, Bxe2x80x2)2O4, A2(B, Bxe2x80x2)O4, A(B, Bxe2x80x2)2O4 and the mixed metal oxides A2BO4, AB2O4, A2Bxe2x80x2O4, ABxe2x80x22O4, Axe2x80x22BO4, Axe2x80x2B2O4, Axe2x80x22Bxe2x80x2O4, and Axe2x80x2Bxe2x80x22O4, where A, Axe2x80x2, B and Bxe2x80x2 are as defined above. In these formulas (A, Axe2x80x2) and (B, Bxe2x80x2) are used to indicate all mixtures of the indicated elements. Presence of second phases can be detected using SEM or TEM methods. These methods and X-ray diffraction analysis can also be employed as known in the art to quantify the amount of second phase(s) present. Crystalline second phases can be detected by X-ray diffraction if they are present at levels of about 1 wt % or more. The amount of second phase detectable by X-ray diffraction depends upon the specific second phase or phases present. SEM and particularly TEM methods can typically be employed to quantify lower amounts of second phases.
Second phases can be added to the MIEC materials by any method. One method is to add the desired second phase or phases to a substantially single-phase MIEC material in powder form, and mix thoroughly to distribute the second phase homogeneously in the mixture prior to pressing and sintering membranes. Another method is to form the second phase or phases simultaneously with the formation of the desired membrane material by mixing an off-stoichiometric ratio of the starting materials.